* The second opengl texture coordinate (gl_TexCoord[1]) are now filled
in as well, and will give canvas coordinates from 0.0 to 1.0. The
first texture coordinates still give the coordinates in the texture
that is being used, which may not match the canvas exactly, so both
coordinates are needed.
* Also optimization to allow using smaller texture sizes with multiple
smaller viewports.
* Print the detailed GLSL shader errors (once), for easier debugging.
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